Electric vehicles are motor-driven vehicles that are equipped with large capacity batteries. In the past, lead storage batteries were used, but these days a nickel hydride batteries and lithium batteries are predominantly used. It is expected that in the futures, lithium batteries will be primarily used.
Lead storage batteries had the advantage of being comparatively inexpensive and highly reliable in the past. However, they are being avoided for usage in electric vehicles because their output per unit is low, their volume is large, and the output voltage declines when used for long periods of time.
Compared to other batteries, lithium batteries are in the spotlight for being high-power and high-density batteries. However, lithium batteries are very expensive, their performance depends largely on temperature, and in particular, the electrolytes decompose at high temperatures, leading to sharply reduced battery lifespans. Moreover, lithium batteries suffer from the risk of ignition and explosion. In order to address these problems, Korean Patent Application Publication Nos. 2010-0001877, 2003-0100891 and 2003-0100893 present methods for cooling batteries.
Currently a lead storage battery of 10 kg can store approximately 1 kWh of electric energy, which can power an electric vehicle to drive 5˜10 km. In order for an electric vehicle to travel 700 km it will require a 1 ton lead storage battery even if a high density lead storage battery is used. Hence a low density battery (i.e. lead storage battery) cannot be used as a secondary battery.
For electric vehicles used for short distance travel of about 100 km with a single charge, a high density battery is not required. Instead a cost efficient lead storage battery may be preferred since a complex cooling system is unnecessary as there is no risk for ignition or explosion. Since there is no risk of ignition and explosion the battery can be installed freely. However, with a long term use the lead storage battery's output voltage will drop and reach a level where it will be difficult to be used for driving. Also, for cases where high output power is needed (i.e. uphill driving or starting from stop) the lead storage battery would not be sufficient, as the output voltage is lower than a lithium battery.